The present invention relates to a solid state light emitting device and, more particularly, to a side surface emission type semiconductor light emitting device.
Semiconductor light emitting elements such as light emitting diodes and laser diodes have greatly contributed to the development of optical fiber communication techniques.
In optical fiber communication systems, the brightness of the light source is set at a desired value by controlling a drive current flowing through the semiconductor light emitting element The light output of the semiconductor light emitting diode is increased upon an increase in drive current. Conversely, when drive current decreases, light output is reduced accordingly. However, the light output of the semiconductor light emitting diode depends not only on drive current but also on a change of temperature or fatigue of the semiconductor light emitting element The light output from the element varies even if the drive current is constant. Recently, an automatic power controller has been used to stabilize the light output. This controller monitors changes in the intensity of light emitted from the semiconductor light emitting element and controls the drive current accordingly.
Conventional semiconductor light emitting elements are classified, in accordance with the direction of light emission, into two groups: edge emission type semiconductor light emitting elements and surface emission type semiconductor light emitting elements. In edge emission type semiconductor light emitting elements, light is emitted in a direction parallel to the pn-junction plane or substrate, and guided to an optical fiber cable. When a light emitting element of this type is optically coupled to the light receiving section of an automatic power controller, no problem occurs. This is because the optical fiber cable is coupled to one of the side surfaces of the semiconductor light emitting element and the light receiving section of the controller faces the other side of the light emitting element. Thus, a light beam emitted from the light emitting element can be directly received by the light receiving section.
In surface emission type semiconductor light emitting elements, a light beam is emitted in a direction perpendicular to the pn-junction plane or substrate, and guided to an optical fiber cable. In order to effectively guide the light beam to the optical fiber cable, reflection in directions other than the cable direction must be prevented. For this reason, when a surface emission type semiconductor light emitting element is optically coupled to the light receiving section of a power controller, an arrangement different from that used for the edge emission type semiconductor light emitting element is necessary. According to a known method, a beam splitter is arranged between the semiconductor light emitting element and the optical fiber cable. The beam splitter guides some of the light beams emitted from the light emitting element to the optical fiber cable, to the light receiving section of the automatic power controller.
Monitoring the light output from the surface emission type element, however, complicates, the structure of the optical system such that the cost of manufacturing a light source unit constituted of a semiconductor light emitting element and a power controller is increased beyond that of a light source using an edge emission type element. Furthermore, a prior art surface emission type element is not positively used due to its low transmission efficiency compared with that of an edge emission type element.